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Scientists who built and will control the instruments to investigate plasma changes around a comet describe their contribution to the ten year long mission at a pre-launch press briefing in London today (Monday 13 January).

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While the actual launch date for the European Space Agency's Rosetta mission has yet to be confirmed, the scientists, engineers and technicians behind the plasma-detecting instruments on board the spacecraft are all ready to begin the journey to comet Wirtanen they hope will return a rich scientific bounty.

"We're not very familiar with plasma here on Earth, but it does exist all around us, for example, in fluorescent lights or the flame of a match. It's simply a gas which has become electrically charged," says Chris Carr, spokesman for the Rosetta Plasma Consortium instruments, based at Imperial College London.

"Outside the confines of our atmosphere on earth, the vacuum of space is filled with a very, very dilute plasma - maybe only a thousand atoms in each litre of space."

The Rosetta Plasma Consortium has built highly sensitive instruments capable of detecting and measuring the properties of this diffuse plasma.

The sensors will be switched on well before the cameras are able to see any activity on the surface of the comet, making it likely that plasma instruments will be one of the first to detect the telltale signature of the comet.

The Plasma Consortium's chief interest is to learn how the solar wind - a stream of plasma that flows out from the Sun and fills the Solar System - interacts with the comet itself.

"A lot of the gas which comes off the comet is actually turned into plasma by the action of the strong ultra-violet light from the Sun," explains Mr Carr.

"So there is a source of plasma pushing outwards from the comet which meets the solar wind head on, producing a 'bubble' of comet plasma in a sea of solar wind."

The plasma instruments will study the structure of this bubble, which measures about a million kilometres wide, and compares with a nucleus size of the comet of just one kilometre.

"One of the things we're really excited about is that we will be monitoring the comet over a long period of time, so we will be able to watch as the comet activity goes from nothing to a really strong outflow of material," says Mr Carr.

The plasma instruments weigh just over 7kg, and because Rosetta is far out in deep space, with very little sunlight shining on the solar panels, have been designed to consume less than a quarter of the power of a single light bulb.

The plasma investigation will be carried out by a group of five instruments built by space researchers from Sweden, Germany, France, USA and the UK.

Scientists at Imperial College London built the Plasma Interface Unit - the 'nervous system' - that links up the five ultra-sensitive plasma-detecting probes aboard Rosetta (See notes to editors).

Assuming a successful Rosetta launch before the end of January 2003, theirs will be the first scientific instrument to be turned on at the 'commissioning' stage due to take place from February at the European Space Agency operations centre in Darmstadt, Germany.

The PIU itself weighs about 3kg and is the size of two shoe boxes on top of each other, and has been the focus of a number of technical innovations.

"Developing this unit, the 'nervous system' for the plasma instruments, was a constant balancing act between miniaturisation to save space and weight and maintaining its reliability to give continuous operation in space for ten years," says Dr Chris Lee, Rosetta Plasma Consortium Operations Manager, based at Imperial College London.

For example, the walls of the box were machined down from sheets of aluminium 2.54 centimetres (an inch) thick to just 0.3mm in places - a machining task that required a new technical innovation from Ray Swain, head of the Department of Physics workshops, as standard techniques left the metal warped.

Scientists from Imperial's Space and Atmospheric Physics Group have extensive experience in building and operating plasma instruments aboard space missions including those that have flown on the Cluster mission around Earth, the Cassini mission to Saturn, the Double Star mission around Earth and the Ulysses mission to the Sun.

The Imperial team behind the PIU was recently promoted from Co-Investigator to Principal Investigator status.

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